Method of coating the inside of a curved tube with fluorescent material



March. 8, 1958 R. w. VAN DER WOLF ETAL 2,827,391

METHOD OF COATING THE INSIDE OF A-CURVED TUBE WITH FLUORESCENT MATERIAL Filed June 20, 1955 INVENTORS mswv W/L LEA/6'5 V4/1/ 05/? W01. F W/L #5. M05 POL XCA/PP 05 0E GPA/9F AGE T United States Patent P ivm'rnon or ooarnso run INSIDE or A cgnvnn rues wirnrLUonnscEN'r MA- at RlAL Rein Willernse van der Wolf and Wilhelmus lolycarpus de Graaf, Eindhoven, Netherlands, assignors, by mesne assignments, to North-American Philips Company, Inc., New York, N. Y., a corporation of Delaware tube is to be used for manufacturing an electric gasand/ or vapour-discharge lamp.

in many gasand/orvapour discharge lamps use is madeof a coating of fluorescent material on the inner wall of the usually tubular envelope, which material converts the radiation from the discharge into radiation of a greater wave-length, for example light. The process of coating the inner wall of such a tube nowadays presents substantially no difiiculty, if the tube is straight. In this event use is generally made of a suspension of the fluorescent material in a solution of an organic binding agent of high molecular weight in a suitable solvent which rapidly evaporates ata lowtemperature. A frequently used method to apply the suspension to the tube which is to be worked up into .a discharge lamp subsequently consists in that the tube is arranged so as to be vertical, af terlwhich the suspension is admitted into its lower end and forced up. .When the suspension has reached the upper end of the tube, the tube is rapidly drained, removed from the used apparatus and dried while verticakfrequently with the use of hot air. This method permits of obtaining asubstantially uniform coating of the inner wall of the tube with the fluorescent material which is bound by the binding agent. Subsequently the binding agent is removed by heating.

in curved tubes, which may not only be in the form of tubes bent so as to becircular but more particularly may be shaped intov more complicated forms, such as are frequently used for advertising signs or decoration, such asirnple methodor" coating the innerwall cannot beused, for in the straight tube it is essential that it is arranged vertical during drying. Thus, the suspension layer adhering to the wall drains to a comparatively high extent. Nevertheless a substantially uniform coating is obtainable by controlling the amount of the binding agent, the viscosity and the temperature during the draining process. In a curved tube this draining is not possible, more particularly when the tube is shaped in a complicated form. Some parts, i. e. the parts which are substantially vertical during draining, would have a substantially uniform coating but other parts would be coated with a materially thicker layer of fluorescent material. This is undesirable not only because a fluorescent layer of varying thickness causes the tube to have a poor appearance but also because the fluorescent material absorbs radiation. Consequently during operation of the lamp the intensity of the radiated light would be widely difierent at different points.

Therefore a plurality of difierent methods have been proposed for coating curved tubes from which electric gas and/or vapour-discharge lamps are to be made.

For a substantially circular tube or a tube which is coiled into a spiral it has been proposed to introduce a limited amount of suspension in the tube and to bring this suspension into contact with the various parts of the Wall by rotating the tube in the vertical plane. The tube 2,82 7,391 Patented Ma 8, 1.9 .8

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'2 is rotatedat so Iowa rate that the tube portion the coating of which is not yet dry is always vertical. This rneth- 0d requires, for example, 1% hours per revolution.

it will be obvious that foratube shaped ,in a complicated form one might start from the same principle, in which event. how v either a wrcqmlicats m ch ne must be used or a worker must bring thetube into the required position manually and slowly change. this-position. Qbviously such a solution of this problem is unacceptable in practice. An alternative solution consists in coating the inner wall .of the tube .with a bindingagent, for example phosphoric acid or glycerin, which retains its binding properties fora long period of time. There.- upon dry fluorescent material is introduced into the tube. This material adheres as a comparatively uniform layer to the wall and the binding agent coated thereon, more particularlywhen the tube is shaken. This methodsuifers from the disadvantage that it is not easy .to distribute the binding agent layer uniformly Over the wall. In some cases use is made of a sponge which is impregnated with the binding agent and is driven through thetube with the aid of compressed air. It has been found thatin this event streaks occur in the direction of, movement of the sponge, i. e. that certain wall portions are, coated with a greater amount of binding agent than other parts.

In order to improve the distribution the, following method has been devised. Small glass beads are used which are coated with a thin layer of binding agent, for example by dipping. The coated beads are introducedinto the tube and brought into contact with the entire inner wall by shaking. It proved possible to obtain. a uniform-coating of the wall in this manner, butrnuch depends onthe skill of the worker carrying out this-method. In addition, frequently ,crackingoccurs, which is.,a heavyloss especially in the case of complicated shapes such as'are required for advertising signs, since the tube has already entirely been formed to have the required shape by a skilled glass blower.

Even if the coating with the binding agent is substantially uniform this method still has .a limitation in that theapplied layer of fluorescent material-has a comparatively open structure. The particles are not closelypacked and consequently the glass wall is exposed to the gas discharge at some points, which causes the tube to be rapidly blackened, more particularly if mercury vapour is used.

In addition, for these methods it isniore diflicult to prepare the fluorescent -materials than for the suspension method.

The invention provides a considerably simplified solution of the problem of coating the inner wall of a tube with fluorescent material, which tube is to be worked up into an electric gasand/ or vapour-discharge lamp. The invention is based on recognition of the fact that even with highly complicated tube shapes substantially always a lar e part of the tube is situated in a level plane. This fact is easily accounted for since most advertising signs and lamps for decorative uses are mounted to walls and ceilings. The parts of the tube which are situated in a level plane need not be adjacent.

According to the invention, a method of coating the inner wall of a curved tube with fluorescent material, which tube is to be worked up into an electric gasand/ or vapour-discharge lamp and comprises a plurality of parts which are situated in a level plane, is characterized in that the tube is filled with a suspension of the fluorescent material in a solution of an organic binding agent having a high molecular weight in a solvent having a boiling point between 30 C. and C. After filling, the excess suspension is removed from the tube and the tube is arranged so that the level plane is horizontal, after which the solvent is removed by reducing the pressure in the tube, which reduction should, however, not be such that the solvent will boil. 7

Although hereinbefore only lamps have been considered it will be obvious that the method in accordance with the invention may also be employed in those cases in which the radiation emitted by the fluorescent layer and produced by the gas discharge, is invisible, for example is situated in the ultra-violet part of the spectrum. The term excess suspension as used herein is to be understood to mean that part of the introduced suspension which does not adhere to the wall.

With the above described horizontal arrangement, in 'the parts of the tube which are situated in the level plane the fluorescent layer will be disposed more remote from 'the viewer, which presents no diificulty.

The more the evaporation of the solution is accelerated the more'uniform the thickness of the layer will be. Consequently it might be proposed to accelerate this evaporation by blowing air through the tube, as is usual in tubes arranged vertically. However, in this'case a force would be exerted on the thin suspension layer adhering to the wall at right angles to the force of gravity. As a result a certain amount of wrinkling and consequently an uneven coating would be unavoidable. Therefore according to the invention the binding agent is rapidly evaporated by reduction of the pressure in the tube. This provides the additional advantage that heat is withdrawn, which causes an additional increase in the viscosity of the suspension with a resulting decrease in sagging. The pressure reduction is obviously readily obtainable by sealing the tube at one end and connecting the other end to. a vacuum pinnp. However, the pressure of the tube should not be reduced to an extent such that the solvent willboil, for otherwise bubble formation and spattering would produce a very uneven luminescent layer comprising many thick and thin portions.

Thus, when lying in a level plane, which level plane is thereafter made v '4 and the like. As suitable solvents we may mention acetone, ethyl acetate, butyl acetate.

The invention will now be described more fully with reference to the accompanying drawing. In this drawing, reference numeral 1 designates a box-shaped fitting in which an apparatus 2 is provided for the generation of a high voltage. This apparatus 2 is supplied from the light mains to which it can be connected by means of a socket 3. On and partially in the box 1 a gasand/or vapour-discharge lamp 4 is arranged which consists of a number of letters and of a number of connecting parts which are shown by broken lines. The letters are disposed on top of the box 1 and the connecting parts in said box. As will be seen from the figure, the parts of a the discharge tube comprising the letters are situated in a level plane. At the ends the discharge tube comprises electrodes 5 and 6 .which are connected to the apparatus 2. If, when carrying out the method in accordance with the invention, the tube, which subsequently is worked up into the discharge lamp 4, after it has been formed to have the shape shown in the figure, is arranged so that the plane of the letters is horizontal, during the sagging process a slightly thicker fluorescent layer will be produced on the bottom of the horizontal parts. However, if the tube is arranged such that the part comprising the thicker fluorescent layer is mounted adjacent the box 1, this will be substantially impercepitble to a viewer of the advertising sign.

What is claimed is: p

l. A method for applying a fluorescent coating to the inner wall of a tube adapted for use in an electric discharge lamp and having a curved portion comprising the steps of, filling the tube with a suspension of fluorescent material in a solution of an organic binding agent having a high molecular weight in a solvent having'a boiling point between 30 C. and 150 C., removingthe excess suspension from the tube, placing at least the curved portion of the tube in a horizontal plane, and reducing the pressure within the tube to thereby remove the solvent but at a value of pressure above that at which the solvent boils.

2..A method as set forth' in claim 1 wherein the curved portion of the tube comprises a plurality of parts References Cited in the file of this patent UNITED STATES PATENTS Taylor Apr; 14, 1953 Kuebler June 30, '1953 

1. A METHOD FOR APPLYING A FLUORESCENT COATING TO THE INNER WALL OF A TUBE ADAPTED FOR USE IN AN ELECTRIC DISCHARGE LAMP AND A CURVED PORTION COMPRISING THE STEPS OF, FILLING THE TUBE WITH A SUSPENSION OF FLUORESCENT MATERIAL IN A SOLUTION OF AN ORGANIC BINDING AGENT HAVING A HIGH MOLECULAR WEIGHT IN A SOLVENT HAVING A BOILING POINT BETWEEN 30*C. AND 150*C., REMOVING THE EXCESS SUSPENSION FROM THE TUBE, PLACING AT LEAST THE CURVED PORTION OF THE TUBE IN A HORIZONTAL PLANE, AND REDUCING THE PRESSURE WITHIN THE TUBE TO THEREBY REMOVE THE SOLVENT BUT AT A VALUE OF PRESSURE ABOVE THAT AT WHICH THE SOLVENT BOILS. 